Printing fluids for printing cellulosic materials of the type referred to above are well known in the prior art. Paper towel printing fluids can utilize dyes or pigments for printing designs on upscale paper towels. Paper towels printed with fluids containing pigments are more difficult to repulp (repulping is the conventional method of recycling paper toweling) than those containing dyes since the pigments are not bleachable. Indeed, the use of printing fluids containing more than 25% pigment results in a dirty base stock upon repulping.
Bleachable dye printing fluids allow higher repulp percentages along with lower raw material costs, but paper towels printed with such fluids bleed badly. The printing fluids of the present invention, however, are unique in that paper towels printed with such fluids do not bleed, yet they are readily bleachable and disappear during the repulping process.
Prior art printing fluids containing dyes are deficient in many respects, particularly bleed resistance, i.e. resistance to print bleeding from the imprinted substrate, e.g. paper toweling, when the substrate is exposed to common household materials, e.g. vinegar, alkaline-type household detergents (e.g. "Formula 409"), water, milk, soft drinks containing citric acid (e.g. "Sprite"). Other problems associated with prior art aqueous printing fluids include poor color strength, poor lightfastness, fiber "picking" from the cellulosic web during the printing process, etc.
Aqueous printing fluids for printing cellulosic materials of the type described above are known in the prior art. U.S. Pat. Nos. 3,864,296 and 3,880,792 disclose aqueous printing fluids and their use in printing cellulosic materials by a rotogravure intaglio printing process. The aqueous printing fluids disclosed in these patents comprise a water-soluble cationic, thermosetting resin and a water-soluble anionic dye which must be compatible with the selected resin. Although these prior art printing fluids impart reasonably good bleed resistance toward water, the bleed resistance of paper printed with such fluids is poor when the imprinted papers are tested with other common household fluids such as milk, vinegar, soft drinks, alkaline based detergents, etc. Moreover, the color strength of such printing fluids is only marginally acceptable.
Although the desired level of bleed resistance may be achieved with pigment-based printing fluids, such fluids are not bleachable and hence, paper towels printed with such fluids are not repulpable. For example paper towels printed with pigment-based printing fluids cannot be bleached and repulped to provide a white paper towel. Any traces of pigment present during the repulping process will remain in the repulped paper towel.